1. Field of the Invention
The present invention relates to a signal processing apparatus, image processing apparatus, and their methods and, for example, to a signal processing apparatus, image processing apparatus, and their methods for converting or transforming (gamut-mapping) a signal in a given color space into that in another color space.
2. Description of Related Art
A computer system or video printer, which has a color monitor and color printer, a color image created and/or processed on a monitor, or a color image received by an image receiving device, is often printed out.
As is known, a color monitor that expresses a color image by emitting light in a specific wavelength range using a phosphor has a quite different color gamut from that of a color printer that expresses a color image by absorbing light in a specific wavelength range using ink or the like, and using remaining reflected light. Furthermore, color monitors have different color gamuts depending on their types, i.e., a monitor that uses liquid crystal, a monitor using an electron gun type cathode ray tube, a monitor using a plasma system, and the like. Similarly, color printers have different color gamuts depending on different paper qualities, different ink consumption amounts, and the like.
For this reason, it is impossible to calorimetrically match the colors of an image displayed on a given color monitor, an image output by a given color printer, and color images output using a plurality of types of printers and a plurality of types of recording sheets. Hence, when a person sees color images displayed (formed) on individual output media, he or she experiences large tincture or color appearance differences among images.
Gamut mapping or color matching is known as an image processing technique for absorbing such perceptual tincture differences of displayed (formed) color images among output media having different color gamuts and perceptually matching their tinctures with each other.
According to one image processing technique, gamut mapping is done by computing the square sum of differences between color signals for all colors used in a color monitor and those used in a color printer, and minimizing the square sum. However, this technique can obtain a satisfactory result within the color gamuts of the two devices, but considerably impair image information for color signals falling outside the ranges due to losses of grayscale characteristics or the like. For this reason, when an image has color signals falling outside the color gamuts of the two devices, it is observed to have quite different colors when it is displayed or formed by these devices.
Also, various gamut mapping techniques have been developed under the condition that grayscale information is preserved as much as possible, in consideration of the aforementioned problem relating to tone reproduction. However, in these techniques, since the amplitude of a difference signal between a color signal of a color monitor and that of a color printer upon expressing a given color becomes large for all colors, images displayed or formed by these devices are observed to have considerably different colors.
In this manner, a gamut mapping technique that can simultaneously solve these two problems is demanded.
As personal computers and workstations prevail, desktop publishing (DTP) and computer-aided design techniques are used in many occasions. In such situation, a color reproduction technique that reproduces colors expressed on a monitor by a computer using actual color agents is important. For example, in case of DTP, a color image is created, edited, processed, and so forth on a color monitor, and is output via a color printer. For this reason, it is strongly demanded that the colors of an image displayed on the color monitor (to be referred to as a “monitor image” hereinafter) perceptually match those of the image printed out (to be referred to as a “print image” hereinafter).
However, perceptual color matching between the monitor and print images encounters difficulty for the aforementioned reasons.
As a technique for absorbing perceptual color differences among output media having different color gamuts and perceptually matching the colors of images formed, gamut mapping for mapping a given color gamut into another color gamut is known. Gamut mapping performs, e.g., linear mapping in the lightness-saturation plane in units of hues in a uniform calorimetric system. According to this technique, the color gamut of a monitor shown in the graph of FIG. 19A is mapped into the color gamut of a printer shown in the graph of FIG. 19B.
However, as shown in FIG. 19B, the mapped color gamut indicated by the solid curve is narrower than the printer color gamut indicated by the one-dashed chain curve. An image that has been corrected by such gamut mapping and is printed out normally has low contrast and poor vividness.